Ethylene-.alpha.-olefin copolymers exhibit excellent characteristics such as heat resistance, weather resistance and ozone resistance and have therefore found broad applications as automobile materials, construction materials, industrial materials and resin modifiers. In particular, ethylene-.alpha.-olefin copolymers having a high ethylene content are soft resins having properties midway between rubbers and crystalline plastics and are now in growing demand as packaging film, etc. Inter alia, copolymers obtained by copolymerizing ethylene and an .alpha.-olefin in the presence of a titanium-based polymerization catalyst are known as linear low-density polyethylene (hereinafter abbreviated as LLDPE) and are widely employed. However, films produced from LLDPE do not always satisfy requirements of low-temperature heat-sealing properties and transparency. The insufficient transparency or heat-sealing properties of the LLDPE are considered attributed to non-uniform composition of ethylene and .alpha.-olefin in the copolymer and broad molecular weight distribution.
On the other hand, several processes for producing ethylene-.alpha.-olefin copolymers using a vanadium-based catalyst have been proposed. For example, JP-B-46-21212 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a process of solution polymerization of ethylene and an .alpha.-olefin using a catalyst system comprised of a vanadium compound and an organoaluminum compound. According to this process, a copolymer is obtained in the form of a uniform solution, i.e., as dissolved in a polymerization solvent. However, the vanadium compound used greatly declines in catalytic activity as the polymerization temperature increases. For example, as shown in the working examples of this patent publication, the polymerization activity becomes too low to be suited for practical use in a high temperature range, e.g., at 100.degree. C, only to produce a copolymer having broad molecular weight distribution and having a high solvent extractable content. JP-B-47-26185 discloses a process for producing an ethylene-.alpha.-olefin copolymer by using a halogenated lower aliphatic hydrocarbon or a hydrocarbon having from 3 to 5 carbon atoms as a polymerization solvent and a combination of a VOX3 compound and an organoaluminum compound as a catalyst system. Polymerization in a halogenated hydrocarbon produces a polymer as a precipitate insoluble in the polymerization solvent, forming a slurry having a low viscosity as a whole. This is economically advantageous in stirring or transporting the system but, in turn, there are problems arising from decomposition of the halogenated hydrocarbon, such as corrosion of apparatus and storage stability of the polymer. In the case of slurry polymerization in a hydrocarbon solvent having 3 to 5 carbon atoms, closeness of the boiling point of this solvent to that of the ethylene-copolymerizable .alpha.-olefin, particularly propylene or 1-butene, gives rise to a great problem in separating the unreacted monomer and the polymerization solvent in the purification step. Further, from an economical viewpoint, this process is not always recognized advantageous since hydrocarbons having 3 to 5 carbon atoms have low boiling points and therefore require a freezing apparatus for liquefication and a pressure-resistant apparatus as well as a cooling medium. Furthermore, the process requires large-sized equipment for an ashing step for removing the catalyst components incorporated into the produced polymer particles.
JP-B-55-24447 discloses a process for producing an ethylene-1-butene copolymer having an ethylene content of from 85 to 95 mol %, in which copolymerization is effected at a temperature of from -20.degree. to 30.degree. C. in an aliphatic hydrocarbon having from 6 to 15 carbon atoms as a polymerization solvent in the presence of a catalyst system composed of a soluble vanadium compound and an organoaluminum halide. Similarly, JP-A-63-17912 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes a process for copolymerizing ethylene and an .alpha.-olefin at from -20.degree. to 30.degree. C. using a catalyst system composed of a soluble vanadium compound and a chlorinated organoaluminum compound. Both of these processes relate to slurry polymerization with a difference lying in that the Al/V atomic molar ratio is from 2/1 to 50/1 in the former process and from 55/1 to 170/1 in the latter process. According to either process, since polymerization is carried out at a relatively low temperature (from -20.degree. to 30.degree. C.), a large quantity of a cooling medium and an energy for driving a freezing device are necessary. In addition, the reaction rate attained is so low that the retention time in the reaction vessel becomes long, increasing the overall volume of the reaction vessel, which results in large consumption of stirring power in the reaction vessel.
In short, when ethylene-.alpha.-olefin copolymers having a high ethylene content are produced by slurry polymerization, that is, in a system in which a part of the copolymer produced is insoluble in a polymerization solvent so that the reaction proceeds while the insoluble copolymer being in a precipitated state, by the conventional processes, it is necessary to make a proper choice of a solvent, and the reaction should be conducted at low temperatures, which is disadvantageous from the standpoint of equipment and energy. On the other hand, in the case of solution polymerization in a system in which copolymerization proceeds while the whole copolymer being dissolved in the solvent, the conventional processes require relatively high temperatures and suffer from reduction of catalyst efficiency, resulting in economical disadvantage.
From all these considerations, it has been keenly demanded to develop a slurry polymerization technique which can be effected at a moderate temperature, more specifically at around 40 to 65.degree. C. at which a vanadium-based catalyst exerts the possible highest activity, which wold be of advantage from the standpoint of equipment, energy, and cost. The most relevant process so far proposed in this connection is found, e.g., in JP-B-46-11028. According to the disclosed technique, however, the resulting copolymer has a largely non-uniform composition and a broad molecular weight distribution, thereby possessing poor strength and poor transparency.